Gas seal

ABSTRACT

A seal for a duct (7) containing a transmission line package (P) comprises a flexible member (2) disposed in a connector (1). The flexible member (2) has a waisted portion that collapses inwardly onto the transmission line package (P) to form a gas seal up to pressures of about 2 bar, above which the waisted portion yields to permit removal or insertion of the transmission line package, for example by the fibre blowing technique. Vents (8) are provided in the connector (1) so that an increase in ambient pressure externally of the connector increases the resistance of the waisted portion to opening. The arrangement may be used to maintain water tightness or pressure differentials over a bulkhead.

This invention relates to a sealing arrangement for a duct, and inparticular to a seal for incorporation in a duct along which an opticalfibre package is installed by fibre blowing.

A duct along which a transmission line (e.g. optical fibre) package isinstalled provides a continuous path along which fluid can travel.Unless such a duct is provided with seals to inhibit fluid flow, hazardscan arise. For example, gas leaks may result in the introduction oftoxic or explosive gases at secondary locations, or water may leakacross otherwise watertight bulkheads. Therefore, to comply with safetyrequirements, after a blown optical fibre installation is completed,seals or blockages are provided at intervals by perforating the duct andinjecting a resin. The disadvantage of this technique is that the resinforms a permanent blockage in the duct, and prevents furtherinstallation of fibre packages, or the replacement of an installedpackage without first removing the resined section of the duct. Removalof the resined section is time-consuming, and requires cutting of theoptical fibre package, requiring subsequent replacement of a completeduct section, or the introduction of additional splices.

The aim of the invention is to provide a seal that does not preventsubsequent installation operations.

The present invention provides a sealing arrangement for a passagewaycarrying a transmission line package, the arrangement comprising acoupling portion for connection to respective passageway portions oneach side thereof, a flexible seal located in the coupling portion andcomprising a flexible body portion, the flexible body portion beingarranged to collapse inwardly to seal around a transmission line packagewhen the transmission line package is installed through the couplingportion, and the flexible body portion being arranged to open to permitfluid flow through the coupling poriton when the fluid flow pressureexceeds a predetermined level, wherein the coupling portion is providedwith at least one vent disposed so that pressure externally of thecoupling portion tends to cause inward deformation of the body portionof the seal to increase its resistance to opening.

Advantageously, the flexible seal comprises a tubular member having awaisted intermediate portion that forms the seal, and wherein the endsof the tubular member are secured to the inner wall of the couplingportion with the or each vent disposed between the locations at whichthe ends of the tubular member are secured. The flexible seal may havesubstantially flat opposing surfaces.

The invention will now be described in greater detail, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic section through a tube connector provided with agas blocking seal according to a preferred embodiment of the invention;

FIG. 2 shows the embodiment of FIG. 1 with a fibre package installed;

FIG. 3 shows the embodiment of FIG. 1 during installation of the fibrepackage;

FIG. 4 shows a modified embodiment during fibre package installation;

FIG. 5 is a schematic section through a duct showing an alternativeembodiment of seal according to the invention in an open condition; and

FIG. 6 is a schematic section through a duct showing the seal of FIG. 5in a sealing condition.

The type of system with which the invention is concerned, and thecontext in which it is described, is in blown fibre installations. Fibreblowing is the process described in our European patent specification108590, in which lightweight transmission lines are urged along ducts bythe viscous drag of a fluid, usually compressed air. The propellant airis typically at an initial pressure of 3 to 10 bar. The transmissionlines typically comprises a bundle of optical fibres encased in alightweight outer covering, and this is termed a fibre package. Fibrepackages may also consist of single fibres or lines, and may includeelectrical conductors.

Referring to the drawings, FIG. 1 shows a tube connector 1 provided witha flexible tubular liner 2 which constitutes a seal. A respectivelocking ring 6 is provided at each end of the connector 1, the lockingrings each receiving and clamping a respective duct section 7 (see FIGS.2 to 4) along which a fibre package P is subsequently blown. The ductsections 7 form part of a duct along which fibre package P is to beinstalled. The ends of the duct extend into the ends of the seal 2,whereby compression of rims 5 on the ends of the connector 1 serves toseal the outer wall of duct sections 7 against the inner wall of theconnector, thereby preventing escape of the installing propellant, oringress of pressure from the external environment.

The central portion of the connector 1 is provided with apertures 8through which the external ambient pressure is communicated to the outerside of the seal 2. The seal 2 itself is moulded to have a waistedshape, the longitudinal section comprising a series of steps and curves.Alternatively, the seal 2 may have a smooth curve or taper from its endsto the centre. The passageway through the centre of the seal 2 is in thenature of a flattened tube, such that it acts as a blockage in theconnector 1 and prevents flow of fluid below a particular pressure e.g.below 2 bar. The tube defined by the seal 2 has a minimum internaldiameter which is less than the diameter of the fibre package P, sothat, once the fibre package is installed, the central part of the sealpresses around the circumference of the fibre package to form agas-tight seal.

The resilience and/or flexibility of the seal 2 is controlled so thatthe seal remains in sealing engagement around the installed fibrepackage P (see FIG. 2) when the external pressure is normal atmosphericpressure, and the pressure within the duct is less than 2 bar (thisvalue may be varied for different applications). If the externalpressure increases, for example due to flooding on one side of abulkhead, the seal 2 is pushed more firmly against the fibre package P,and increases the resistance of the seal to opening, so that, if therewas a fracture in the duct section 7 on the high pressure (flooded) sideof the seal, the duct would not provide a breach of the bulkhead becausethe same additional pressure maintains the seal in the closedconfiguration.

FIG. 3 shows how the seal 2 may deform in order to permit installation.Air, or some other propellant, is introduced into the duct at a pressurein excess of that required to deform the seal. Consequently, the sealexpands rather like a balloon or inner tube, and the propellant and thefibre package P (which is urged along by the propellant) pass throughthe opened seal 2, which closes around the package once the introductionof pressurised propellant is discontinued. The seal 2 (see FIG. 3)deforms by flexure of the central waisted portion in the direction offlow. In an alternative embodiment (see FIG. 4), the central waistedportion of the seal 2 flexes radially outwardly. The geometry of thedeformation may be aided or controlled by the provision of selectiveareas or lines of weakness or stiffening (not shown), along whichflexure or resistance to flexure will preferentially occur.

FIGS. 5 and 6 shown an alternative embodiment, in which a duct 11 isprovided with an insert seal 12 which comprises an annular flangeportion 12a and a body portion 12b. The flange portion 12a engagesbetween two connecting sections of the duct 11, and is formed as part ofthe connector 15 that holds the duct section together. The connector 15is not shown in detail in FIGS. 5 and 6, but is similar to the connector1 of FIGS. 1 to 4. Alternatively, the flange portion 12b of the seal 12is adapted to be held by the connector 15. The body portion 12b is madeof a relatively-soft, flexible material such as a synthetic rubber, andmay have a variety of configurations. For example, it may be tubularwith a bore diameter substantially equal to, or less than, the minimumdiameter of the transmission line package P that is to be installedalong the duct 11.

The bore of the tubular body portion 12b may be conical or tapering,with the wider end connected to the flange portion 12a. Alternatively,the tubular body portion 12b may comprise a flat body portion consistingessentially of upper and lower lip portions that press against eachother in the undisturbed state.

When at rest, either before or after installation of the transmissionline package P along the duct 11 the body portion 12b is collapsedinwardly as shown in FIG. 6. When the transmission line package P ispresent the tubular body portion 12b forms a seal around the package Pwhich is sufficiently close-fitting to prevent substantive egress (orlow pressure flow) of gas along the duct 11. The elasticity of the seal12 is such that, from one direction, a relatively high pressure flow ofgas, such as 100 psi, will expand the seal sufficiently to enablepassage of the package P (see FIG. 5). Preferably, the seal 12 expandsto lie substantially flat against the sides of the duct 11, therebypresenting a minimum impedance to the passage of the package P during ablowing installation. Once the package P is installed, and the injectionof compressed gas into the duct 11 has ceased, the body portion 12b ofthe seal 12 collapses back onto the fibre package.

The seal 12 described above with reference to FIGS. 5 and 6 isunidirectional, in that it permits fibre blowing installation in onedirection, but would remain resistant to opening in the oppositedirection. It is envisaged that, by providing a sufficiently-thin seal,rather like a diaphragm with a central, short-lipped aperture, a sealthat will open bidirectionally can be provided, with the lips invertingthrough the aperture to permit either direction of high pressure flow.

Unidirectionally-opening seals may be made to have high resistance inone direction, so that blown installation can take place only from onedirection, thus preventing unauthorised installation from the otherdirection. This may be particularly useful for maintaining integritywithin a building, with it being possible to blow packages outwardlyfrom the building, but preventing access or even high pressure fluidflow along the ducts from outside the building.

Seals of the type described may be provided at intervals along a routeto prevent low pressure leakage of contaminants along the duct. Anotherapplication is to provide such a seal in a lead-through gland betweentwo airtight chambers, for example in the watertight bulkhead of asea-going vessel. In this latter application, a seal connection, such asthat shown in FIG. 2, is preferably provided each side of a bulkhead, sothat a leak on either side causes additional sealing by water pressureas previously described.

I claim:
 1. A sealing arrangement for a passageway carrying atransmission line package, the arrangement comprising a coupling portionfor connection to respective passageway portions on each side thereof, aflexible seal located in the coupling portion and comprising a flexiblebody portion, the flexible body portion being arranged to collapseinwardly to seal around a transmission line package when thetransmission line package is installed through the coupling portion, andthe flexible body portion being arranged to open to permit fluid flowthrough the coupling portion when the fluid flow pressure exceeds apredetermined level, wherein the coupling portion is provided with atleast one vent disposed so that pressure externally of the couplingportion tends to cause inward deformation of the body portion of theseal to increase its resistance to opening.
 2. A sealing arrangement asclaimed in claim 1, wherein the flexible seal comprises a tubular memberhaving a waisted intermediate portion that forms the seal, and whereinthe ends of the tubular member are secured to the inner wall of thecoupling portion with the or each vent disposed between the locations atwhich the ends of the tubular member are secured.
 3. A sealingarrangement as claimed in claim 1, wherein the flexible seal hassubstantially flat opposing surfaces.
 4. A sealing arrangement asclaimed in claim 1, wherein the seal opens unidirectionally.
 5. Ablown-fibre installation comprising at least one blown-fibre duct whichextends outside a building from within the building, a gas seal beingprovided in a segment of said duct to prevent the flow of gas into thebuilding via said duct, characterised in that said gas seal is flexibleand comprises a flexible body portion, the flexible body portion beingarranged to collapse inwardly around a transmission line package whenthe transmission line has been installed through said duct segment, theflexible body portion being arranged to open to permit fluid flowthrough said duct segment when the fluid flow pressure exceeds apredetermined level, and wherein the flexible body portion extends alongsaid duct segment and is arranged to form an elongate seal with thesurface of an installed transmission line package.
 6. A blown-fibreinstallation as claimed in claim 5, wherein a transmission line packageextends through said seal.
 7. An installation as claimed in claim 6wherein said flexible body portion sealingly conforms to the surface ofsaid transmission line package.